The solar system was a cramped, provincial place in May 1989, and so was the Italian restaurant in downtown Baltimore where Alan Stern exhorted a dozen of his colleagues to join him on an improbable mission: Persuade NASA to send a probe to Pluto. At the time, even most astronomers weren't interested in this weird, tiny orb on the fringe of the solar system. "I was skeptical," says Fran Bagenal, then a graduate student at MIT, who was present at the meal. "Sending a mission to Pluto seemed pretty uninteresting at the time."
While Bagenal picked at her pasta, Stern waxed on about Pluto's charms. He explained his research, analyzing how the solar wind—the continuous flow of particles exhaled by the sun—blows gases from Pluto's surface like seeds from a dandelion. "I always liked to study things that were somewhat offbeat," Stern says. Pluto definitely fits—a world smaller than Earth's moon, stuck in an oval orbit tilted at a rakish angle. Stern was fascinated that Pluto in many ways resembles an overgrown comet more than an undersized planet. Its surface is covered with frozen gases that evaporate and form an atmosphere when its orbit approaches the sun. Its gravity is so weak that much of that atmosphere streams off into space, like the tail of a comet. In the 4.5-billion-year history of the solar system, whole mountain ranges and craters on Pluto's surface have simply vaporized.
Stern pointed out that every other planet in the solar system had been visited by a probe or was about to be. Each mission yielded astonishing revelations: Venus is a hellish inferno, Mars is dotted with craters and crossed with ancient channels, Jupiter is surrounded by moons of fire and ice. The more Bagenal listened, the more interested she found herself becoming: "You begin to get curious about this little thing at the edge of the solar system."
It soon became clear that Stern was prescient. Just three years after he gave his pitch in Baltimore, David Jewitt and Jane Luu of the University of Hawaii discovered QB1, a distant planetoid that resembled a smaller version of Pluto.
Since then, astronomers have cataloged more than 1,000 sizable objects in that same zone, orbiting in the dim outer reaches of the solar system. At least one of them, discovered last year and nicknamed Xena, is larger than Pluto, has a major moon, and seems to qualify as a 10th planet. Far from being an oddity, Pluto is now seen as the most accessible object in the most interesting place in the solar system: the recently discovered Kuiper belt, a system of icy worlds that extends 19 billion miles or so beyond Pluto.
"Our view of the solar system has been radically transformed," Stern says. "If you grew up in a mountain valley and thought that the world ended where that little ring of mountains is, you'd have no idea how big the world is. The Kuiper belt is the largest structure in the planetary solar system. It's where most of the planets are, where most of the organic chemicals are, where double planets and other exotic things are. We never had a clue about that. When I was going to school, Pluto was a misfit. Now it looks like Earth is the misfit."
For Stern, the sweetest vindication is that he finally sold NASA on his vision. New Horizons, the spacecraft he has dreamed and schemed about for 17 years, should now be on its way to Pluto and beyond, into the Kuiper belt. When it arrives, Stern and the rest of us will finally start to understand the other half of the solar system.
With all the years that have passed since the original dozen scientists—Stern calls them the Pluto Underground—got together to brainstorm the mission, one would think that New Horizons was the product of steady, meticulous planning. In fact, it is a rather hurried mission that got the final green light only three years ago.
The rush was dictated in part by Pluto's orbit. When Stern and the Pluto Underground were sitting down to dinner in 1989, the planet had reached its near point to the sun at about 2.7 billion miles and had begun receding toward the far end of its orbit, about 4.5 billion miles out. Pluto takes 248 years to circle the sun, so as it moves farther out into space, it gets cooler with each passing day. At some point soon, probably in the next decade or two, the atmosphere will freeze and condense into crystals on the surface. We are quickly losing a once-in-a-lifetime opportunity to examine the planet's most interesting features.
Urgency also comes from the shifting arrangement of the planets, which in Pluto's case makes it steadily more difficult to fling a spacecraft all the way out there. A January launch allows the ship to sail within 1.4 million miles of Jupiter in early 2007, where it will receive a gravitational boost that increases its velocity to 13 miles per second. Even at that speed, New Horizons will not reach Pluto until 2015. A delayed launch just a month later, in February, would mean bypassing Jupiter and a journey that takes four years longer.
While Pluto was moving on, NASA was dithering. Way back in 1979, NASA considered sending its Voyager 1 spacecraft past Pluto, but that plan was abandoned as too costly and logistically impractical. Lingering disappointment over that decision helped inspire Stern to form the Pluto Underground. He wanted to send two ships toward Pluto, doubling up the mission in order to see both sides of the planet. Any probe that has to travel such a great distance has to build up a lot of speed, so each ship would see only one hemisphere of Pluto as it flew by. Slowing down the ship to take a leisurely look would not be an option—it requires far too much fuel.
Almost immediately after the dinner in 1989, Stern and the Pluto Underground started lobbying NASA program administrators. It became a maddening on-again, off-again process that dragged on for more than a decade. NASA officials initiated a feasibility study of Stern's plan, but the effort seemed to go nowhere. Unable to provoke firm logistical or financial support, Stern dropped out of the planning process. A separate group at the Jet Propulsion Laboratory proposed sending a single small, cheap probe, but when they worked out the details, the projected cost swelled to more than $1 billion. "Somebody said, 'Look, this is bonkers,' " says Bagenal, now a planetary scientist at the University of Colorado at Boulder. "Even I tended to agree. It wasn't worth it."
What ultimately resurrected the Pluto mission was the discovery of other worlds in the Kuiper belt, sparking a lively debate over whether Pluto really was a planet. Astronomy buffs wrote to NASA asking why the United States had no plans to explore the solar system's Wild West. In 2001 NASA officials decided to revisit the idea of a Pluto mission and this time opened it up to competitive bidding. A team at the Jet Propulsion Laboratory tried to recruit Stern, but he feared the administrators there might not devote their best engineers and scientists to the effort. So he joined up with a collaboration headquartered at the Applied Physics Laboratory at Johns Hopkins University, known for small, innovative projects like the NEAR probe, which eventually performed a soft landing on the asteroid Eros in 2001.
Stern had been working most of his professional life to get a Pluto mission off the ground. He had advanced in his field and become director of space studies for the Southwest Research Institute in Boulder, Colorado. And he knew exactly what he wanted. One afternoon he took a single piece of paper and wrote down everything a Pluto mission should be. The biggest challenge was affordability. NASA set the budget at $500 million—not much for a 4-billion-mile 10-year trip. "Everything had to be the safest, the surest bet," Stern says. "NASA would be looking for low risk. We had to be at the church on time, and on budget."
Stern's team, which included Bagenal along with Andy Cheng and Ralph McNutt, physicists at Johns Hopkins, called for a simple, lightweight ship packing seven science experiments into a compact, eight-foot-wide package. New Horizons has two cameras to capture images of Pluto and its largest moon, Charon. The spacecraft also includes a particle detector designed to measure the interactions between Pluto's atmosphere and the solar wind. And it carries a radio detector to monitor signals from Earth as they graze past Pluto's surface; their distortion will document the composition and structure of the atmosphere. Solar power is impractical that far from the sun, so NASA decided on a small plutonium-powered reactor. The whole package weighs slightly more than 1,000 pounds and runs on less than 200 watts, about the amount consumed by a bright floor lamp.